U.S. patent number 6,449,262 [Application Number 09/286,993] was granted by the patent office on 2002-09-10 for method and apparatus for frequency shifting with a clock signal.
This patent grant is currently assigned to Legerity. Invention is credited to Maged F. Barsoum, Hungming Chang, Eugen Gershon, Muoi V. Huynh, Chien-Meen Hwang.
United States Patent |
6,449,262 |
Hwang , et al. |
September 10, 2002 |
Method and apparatus for frequency shifting with a clock signal
Abstract
A physical layer device (PHY) device in a home LAN employs
discrete multitone technology (DMT). The DMT system enables usage
of existing residential wiring, which typically is telephone system
grade twisted copper pair. The PHY device comprises an analog front
end (AFE) circuit that frequency shifts the spectral images by
using a clock signal. The multiplication of the clock signal is
accomplished using a digital mixer or, in the alternative, an
analog switch.
Inventors: |
Hwang; Chien-Meen (San Jose,
CA), Huynh; Muoi V. (San Jose, CA), Barsoum; Maged F.
(Sunnyvale, CA), Chang; Hungming (Cupertino, CA),
Gershon; Eugen (San Jose, CA) |
Assignee: |
Legerity (Austin, TX)
|
Family
ID: |
23101020 |
Appl.
No.: |
09/286,993 |
Filed: |
April 7, 1999 |
Current U.S.
Class: |
370/307; 370/430;
370/478; 370/481; 375/222 |
Current CPC
Class: |
H04L
27/0002 (20130101); H04L 27/2626 (20130101); H04L
27/2647 (20130101) |
Current International
Class: |
H04L
27/00 (20060101); H04L 27/26 (20060101); H04J
004/00 () |
Field of
Search: |
;375/303,260,295,298,340,222,233,283,130,261,265,291,285
;370/203,204,206,207,210,493,495,307
;307/208,209,430,477,478,480,481,503,516,518 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
5537435 |
July 1996 |
Carney et al. |
5541955 |
July 1996 |
Jacobsmeyer |
5666378 |
September 1997 |
Marchetto et al. |
6192068 |
February 2001 |
Fattouche et al. |
|
Primary Examiner: Cangialosi; Salvatore
Attorney, Agent or Firm: McDermott, Will & Emery
Claims
What is claimed is:
1. A communication system for transmitting a bit stream,
comprising: a transmitter circuit for generating a symbol
comprising differentially encoded signals, each of the
differentially encoded signals being mapped to one of a plurality
of carrier frequencies based upon the bit stream; a receiver
circuit for receiving the symbol and decoding the differentially
encoded signals of the symbol to output the bit stream, the
received differentially encoded signals each having a plurality of
spectral images associated with the corresponding carrier frequency
and harmonics of the corresponding carrier frequency; and a system
clock coupled to the transmitter circuit, wherein the transmitter
circuit includes circuitry for frequency shifting the
differentially encoded signals based upon the system clock
rate.
2. The communication system as in claim 1, wherein the transmitter
circuit comprises: a differential encoder for mapping the bit
stream into corresponding differentially encoded frequency-domain
signals, each of the differentially encoded frequency-domain
signals being equated to N bits of the bit stream, wherein N is an
integer; frequency-to-time transform logic for converting in a
parallel manner each of the differentially encoded frequency-domain
signals to time-domain signals; parallel-to-serial logic for
converting the time-domain signals to a serial stream of
differentially encoded signals; and an analog front end (AFE)
circuit for selectively filtering the differentially encoded
signals of the symbol to pass a prescribed number of spectral
images, performing digital to analog conversion of the filtered
serial stream based upon the system clock rate, and subsequently
transmitting the symbol based upon the system clock rate.
3. The communication system as in claim 2, wherein the AFE circuit
comprises a digital mixer for frequency shifting the differentially
encoded signals in response to the system clock rate.
4. The communication system as in claim 3, wherein the system clock
rate is twice the frequency of the carrier frequency.
5. The communication system as in claim 2, wherein the AFE circuit
includes the circuitry for frequency shifting the differentially
encoded signals based upon the system clock rate, the circuitry
comprising an analog switch.
6. The communication system as in claim 1, wherein the receiver
circuit comprises: an analog front end circuit (AFE) for receiving
the symbol and performing analog to digital conversion of the
differentially encoded signals; serial-to-parallel logic for
converting the differentially encoded signals to a parallel array
of differentially encoded signals; time-to-frequency transform
logic for converting each of the parallel differentially encoded
signals to frequency-domain signals; and differential decoder logic
for mapping each of the frequency-domain signals to a corresponding
N bits.
7. The communication system as in claim 2, wherein N is equal to 2
and the plurality of carrier frequencies is 256.
8. The communication system as in claim 1, wherein the symbol
further comprises a cyclic prefix to provide symbol separation.
9. The communication system as in claim 2, wherein the
frequency-to-time transform logic executes an inverse Fast Fourier
Transform.
10. The communication system as in claim 6, wherein the
time-to-frequency transform logic executes a Fast Fourier
Transform.
11. A method for transmitting a bit stream, comprising the steps
of: generating a symbol comprising differentially encoded signals
based upon the bit stream via a plurality of carrier frequencies;
receiving the symbol and decoding the differentially encoded
signals of the symbol to output the bit stream, the received
encoded signals each having a plurality of spectral images
associated with the corresponding carrier frequency and harmonics
of the carrier frequency; and frequency shifting the differentially
encoded signals based upon a clock signal.
12. The method as in claim 11, wherein the step of frequency
shifting is performed by a digital mixer configured for operating
in response to the clock rate of the clock signal.
13. The method as in claim 11, wherein the step of frequency
shifting is performed by an analog switch configured for operating
in response to the clock rate of the clock signal.
14. The method as in claim 11, wherein the step of generating
further comprises: filtering the symbol to pass a prescribed number
of spectral images; transforming the differentially encoded signals
from frequency-domain signals to time-domain signals; creating a
serial stream of the time-domain signals; and performing digital to
analog conversion of serial stream based upon the clock rate of the
clock signal.
15. The method as in claim 14, wherein the step of transforming is
performed by an inverse Fast Fourier Transform logic.
16. The method as in claim 11, wherein the step of receiving
comprises: filtering the received symbol; performing analog to
digital conversion of the received differentially encoded signals
of the symbol based upon the clock rate of the clock signal;
converting the digitized differentially encoded signals from serial
signals to parallel signals; and transforming the differentially
encoded signals into frequency-domain signals.
17. The method as in claim 16, wherein step of transforming is
performed by a Fast Fourier Transform logic.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discrete multi-tone (DMT) data
communications network, more particularly to a DMT transceiver
device.
2. Description of the Related Art
Modern society continues to create exponentially increasing demands
for digital information and the communication of such information
between data devices. Local area networks use a network, cable or
other media to link stations on the network for exchange of
information in the form of packets of digital data. A typical local
area network architecture uses a media access control (MAC)
enabling network interface cards at each station to share access to
the media. Most conventional local area network architectures use
media access controllers operating according to half-duplex or
full-duplex Ethernet (ANSI/IEEE standard 802.3) protocol and a
prescribed network medium, such as twisted pair cable.
These architectures have proven quite successful in providing data
communications in commercial applications. However, these common
local area network architectures require installation of
specialized wiring and use of specific wiring topologies. For
example, the most popular network protocols, such as Ethernet,
require special rules for the wiring, for example with regard to
quality of wire, range of transmission and termination.
Due to the success of the Internet and the rapid decreases in the
prices of personal computers and associated data equipment, a
demand has arisen for data communications between a limited number
of devices within relatively small premises, typically a residence
or small business. While existing local area networks can serve the
purpose, in such installations, the cost of installing physical
network wiring satisfying the rules for the particular protocol can
be prohibitively expensive.
Most existing buildings, including residences, include some
existing wiring, for phones, electrical power and the like.
Proposals have been made to communicate data using such existing
infrastructure. This reduces the costs of wiring for the network,
but the existing wiring raises a variety of issues regarding
transport of high-speed digital signals.
For example, efforts are underway to develop an architecture that
enables computers to be linked together using conventional twisted
pair telephone lines. Such an arrangement, referred to herein as a
home network environment, provides the advantage that existing
telephone wiring in a home may be used to implement a home network
environment without incurring costs for substantial new wiring
installation. However, any such network must deal with issues
relating to the specific nature of in-home telephone wiring, such
as operation over a media shared with other services without
interference from or interfering with the other services, irregular
topology, and noise. With respect to the noise issue, every device
on the telephone line may be a thermal noise source, and the wiring
may act much like an antenna to pick up disruptive radio signal
noise. Telephone lines are inherently noisy due to spurious noise
caused by electrical devices in the home, for example dimmer
switches, transformers of home appliances, etc. In addition, the
twisted pair telephone lines suffer from turn-on transients due to
on-hook and off-hook and noise pulses from the standard telephones
coupled to the lines, and electrical systems such as heating and
air conditioning systems, etc.
An additional problem in telephone wiring networks is that the
signal condition (i.e., shape) of a transmitted waveform depends
largely on the wiring topology. Numerous branch connections in the
twisted pair telephone line medium, as well as the different
associated lengths of the branch connections, may cause multiple
signal reflections on a transmitted network signal. Telephone
wiring topology may cause the network signal from one network
station to have a peak-to-peak voltage on the order of 10 to 20
millivolts, whereas network signals from another network station
may have a value on the order of one to two volts. Hence, the
amplitude and shape of a received pulse may be so distorted that
recovery of a transmit clock or transmit data from the received
pulse becomes substantially difficult.
At the same time a number of XDSL technologies are being developed
and are in early stages of deployment, for providing substantially
higher rates of data communication over twisted pair telephone
wiring of the telephone network. XDSL here is used as a generic
term for a group of higher-rate digital subscriber line
communication schemes capable of utilizing twisted pair wiring from
an office or other terminal node of a telephone network to the
subscriber premises. Examples under various stages of development
include ADSL (Asymmetrical Digital Subscriber Line), HDSL (High
data rate Digital Subscriber Line) and VDSL (Very high data rate
Digital Subscriber Line).
Consider ADSL as a representative example. For an ADSL-based
related service, the user's telephone network carrier installs one
ADSL modem unit at the network end of the user's existing
twisted-pair copper telephone wiring. Typically, this modem is
installed in the serving central office or in the remote terminal
of a digital loop carrier system. The user obtains a compatible
ADSL modem and connects that modem to the customer premises end of
the telephone wiring. The user's computer connects to the modem.
The central office modem is sometimes referred to as an ADSL
Terminal Unit--Central Office or `ATU-C`. The customer premises
modem is sometimes referred to as an ADSL Terminal Unit--Remote or
`ATU-R`. The ADSL user's normal telephone equipment also connects
to the line through a frequency combiner/splitter, which is
incorporated in the ATU-R. The normal telephone signals are split
off at both ends of the line and processed in the normal
manner.
For digital data communication purposes, the ATU-C and ATU-R modem
units create at least two logical channels in the frequency
spectrum above that used for the normal telephone traffic. One of
these channels is a medium speed duplex channel; the other is a
high-speed downstream only channel. Two techniques are under
development for dividing the usable bandwidth of the telephone line
to provide these channels. One approach uses Echo Cancellation.
Currently, the most common approach is to divide the usable
bandwidth of a twisted wire pair telephone line by frequency, that
is to say by Frequency Division Multiplexing (FDM).
FDM uses one frequency band for upstream data and another frequency
band for downstream data. The downstream path is then divided by
time division multiplexing into one or more high-speed channels and
one or more low speed channels. The upstream path also may be
time-division multiplexed into corresponding low speed
channels.
The FDM data transport for ADSL services utilizes discrete
multi-tone (DMT) technology. A DMT signal is basically the sum of N
independently QAM modulated signals, each carried over a distinct
carrier frequency channel. The frequency separation between
consecutive carriers is 4.3125 kHz with a total number of 256
carriers or tones (ANSI). An asymmetrical implementation of this
256 tone-carrier DMT coding scheme might use tones 32-255 to
provide a downstream channel of approximately 1 MHz analog
bandwidth. In such an implementation, tones 8-31 are used as
carriers to provide an upstream channel of approximately 100 kHz
analog bandwidth. Each tone is quadrature amplitude modulated (QAM)
to carry up to 15 bits of data on each cycle of the tone waveform
(symbol).
A conventional DMT system is shown in FIG. 6. The transmitter 601
includes a constellation point mapper 603 for logically mapping
input bit streams onto a complex plane, whereby each sequence of
bits (e.g., 2 bits) is equated to a complex number (i.e.,
constellation point). A constellation point represents the
amplitude and phase of a particular tone. A typical ADSL system,
for instance, employs 256 tones. An inverse Fast Fourier transform
(IFFT) 605 then converts the constellation points, which provide
information in the frequency-domain, to time-domain waveforms for
transmission over the channel 625. Each conversion transforms 256
constellation points (complex numbers) into 512 samples of the time
domain waveform. A parallel-to-serial block 607 clocks the samples
out in a serial sequence for input to the analog front end (AFE)
block 609, which is described below in the discussion of FIG. 7.
The AFE block 609 outputs the actual bandpass waveform that is
transmitted across the channel 625.
On the receiver side, the bandpass signal enters the receive AFE
block 613. The AFE block 613 outputs a serial sequence of the
digitized received waveforms to the input of the serial/parallel
block 613, which converts the serial stream into a parallel set of
data. The parallel data is then input into a Fast Fourier transform
(FFT) 617 to extract the corresponding frequency-domain signals.
The resulting frequency-domain data may display spectral power loss
mainly because of the channel attenuation and digital to analog
(D/A) conversion. Accordingly, the received signals usually undergo
equalization to restore their spectral energy distributions. Slicer
621 then performs decoding of the complex numbers to corresponding
bit streams.
FIG. 7 illustrates a traditional transmitter side AFE block 609
employed in the DMT system of FIG. 6. This transmitter side AFE
block 609 comprises essentially four basic components: a D/A
converter 701, low pass filter (LPF) 703, mixer 705, and a voltage
controlled oscillator 707. The digital waveforms from the IFFT 605
are converted to an analog waveform (i.e., baseband signal). The
baseband signal is fed into the LPF 703 to eliminate unwanted high
frequencies; a typical cutoff frequency of the LPF 703 is 138 kHz.
The filtered baseband signal is then up converted by mixer 705; the
voltage controlled oscillator (VCO) 707 supplies a sinusoidal
signal with an amplitude of A and frequency of co to the mixer 705.
The mixer 705 and the VCO 707 operator as a modulator. Use of a VCO
to directly frequency shift the modulating signal poses
implementation constraints in terms of flexibility of integrated
circuit (IC) design.
The AFE block 613 for the receiver side performs the above
operations in essentially the reverse sequence. That is, the AFE
block 613 receives the bandpass signal from the channel 625 and
down converts it to restore the baseband signal. The baseband
signal is then input to a LPF and then digitized with an analog to
digital (A/D) converter (not shown).
The existing DSL systems provide effective high-speed data
communications over twisted pair wiring between customer premises
and corresponding network-side units, for example located at a
central office of the telephone network. The DSL modem units
overcome many of the problems involved in data communication over
twisted pair wiring. However, for a number of reasons, the existing
DSL units are not suitable to providing local area network type
communications within a customer's premises. For example, existing
ADSL units are designed for point-to-point communication. That is
to say, one ATU-R at the residence communicates with one ATU-C unit
on the network end of the customer's line, and the units are not
useable for multi-point communications. Also, existing ADSL modems
tend to be quite complex, and therefore are too expensive for
in-home communications between multiple data devices of one
customer. A need therefore still exists for techniques to adapt DMT
type DSL communications for use in an in-home multi-point
environment.
SUMMARY OF THE INVENTION
There exists a need for a DMT system that is tailored for use over
existing in-home wiring. In particular, the DMT system needs to
provide a technique for frequency shifting that is flexible in
terms of design choice and can be readily implemented without
complex logic or circuitry.
These and other needs are satisfied by the present invention, where
a communication system includes a transmitter circuit that outputs
a symbol represented by differentially encoded signals over a range
of frequencies (or tones). The transmitter utilizes the system
clock to frequency shift the differentially encoded signals for
transmission across the channel.
According to one aspect of the present invention, a communication
system for transmitting a bit stream comprises a transmitter
circuit that generates a symbol. The symbol includes differentially
encoded signals, in which each of the differentially encoded
signals is mapped to one of a plurality of carrier frequencies
based upon the bit stream. A receiver circuit receives the symbol
and decodes the differentially encoded signals portion of the
symbol to output the bit stream. Each of the received
differentially encoded signals has a plurality of spectral images
associated with the corresponding carrier frequency and its
harmonics. A system clock is coupled to the transmitter circuit for
frequency shifting the differentially encoded signals based upon
the clock rate of the system clock. This arrangement advantageously
permits an efficient technique of up converting the baseband
symbol.
Another aspect of the present invention provides a method for
transmitting a bit stream. The method comprises generating a symbol
comprising differentially encoded signals based upon the bit stream
via a plurality of carrier frequencies. The method also includes
receiving the symbol and decoding the differentially encoded
signals of the symbol to output the bit stream. Each of the
received encoded signals has a plurality of spectral images
associated with the corresponding carrier frequency and its
harmonics. Further, the method includes frequency shifting the
differentially encoded signals based upon a clock signal. The above
method provides a cost-effective implementation of modulating the
symbol.
Additional advantages and novel features of the invention will be
set forth in part in the description which follows, and in part may
become apparent to those skilled in the art upon examination of the
following or may be learned by practice of the invention. The
advantages of the invention may be realized and attained by means
of the instrumentalities and combinations particularly pointed out
in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a DMT system according to an
embodiment of the present invention.
FIG. 2 is a block diagram of the transmitter side AFE circuit of
the DMT system of FIG. 1.
FIG. 3 is a diagram illustrating the concept of differential
encoding in a DMT system in accordance with an embodiment of the
present invention.
FIGS. 4a and 4b are block diagrams of the transmitter side AFE
circuit utilizing a system clock to frequency shift in accordance
with the embodiments of the present invention.
FIG. 5 is a spectrum of an exemplary symbol that has been frequency
shifted in accordance with an embodiment of the present
invention.
FIG. 6 is a block diagram of a conventional DMT system.
FIG. 7 is a block diagram of a conventional AFE circuit of the DMT
system of FIG. 6.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The present invention contemplates providing frequency shifting of
encoded signals in a DMT system, such as a DMT transceiver that
utilizes differential encoding. It will become apparent, however,
that the present invention is also applicable to other types of
communication systems and devices.
FIG. 1 is a block diagram of a DMT system in which the present
invention may be advantageously employed. A transmitter circuit 101
communicates with a receiver circuit 111 over a physical channel
125, which in an exemplary embodiment is a twisted pair
infrastructure. In the implementation of a DMT transceiver (not
shown), the transmitter circuit and the receiver circuit both
reside on a single printed circuit board. Consequently, two DMT
transceivers would be required to communicate across the channel
125. For purposes of explanation, FIG. 1 simply shows a transmitter
circuit 101 sending data signals to a receiver circuit 111.
The transmitter 101 receives a digital bit stream from a digital
source, for example, a host central processing unit (CPU) (not
shown). The bit stream enters the differential encoder 103, which
maps bit sequences of the bit stream to points on the complex plane
(i.e., constellation points), in similar fashion to the operating
principles of QPSK (quaternary phase shift keying). However, unlike
QPSK, the differential encoder 103 encodes the difference between a
current constellation point with a reference constellation point.
The phase difference between constellation points represents two
bits. The reference point is transmitted in a reference symbol
prior to sending the symbols that contain actual data bits. In an
exemplary embodiment, the differential encoder 103 encodes a bit
stream in bit sequences of 2-bits using 256 frequencies (or tones);
as a result, a symbol represents 512 bits. Because the encoding
scheme focuses on amplitude and phase differences, absolute signal
values are not needed for correctly detecting and decoding the
received symbols. As such, this technique eliminates the need for a
frequency domain equalizer (FEQ) to compensate for amplitude and
phase distortion caused by the channel 125. The concept of
differential encoding is more fully described below in the
discussion of FIG. 3.
The output of the differential encoder 103 is a parallel array of
"complex numbers." These complex numbers are input in a parallel
manner to an Inverse Fast Fourier Transform (IFFT) logic 105. The
IFFT logic 105 converts the complex numbers into time-domain
waveforms, which are then supplied to an output parallel-to-serial
logic 107. The output parallel-to-serial logic 107 basically
arranges the time-domain waveform into a serial stream of samples.
A guard band or cyclic-prefix can be prepended to this serial
stream before entering the analog front end (AFE) circuit 109 to
minimize intersymbol interference (ISI). ISI is an overlapping of
waveforms that distorts all symbols involved, potentially resulting
in data loss. The AFE circuit 109 is discussed in more detail with
respect to FIG. 2. Among other functions, the AFE circuit 109
performs digital to analog (D/A) conversion and sends the symbol,
which includes the differentially encoded signals, over channel 125
to receiver 111.
As seen in FIG. 1, after processing of the received signal by the
AFE 113, the digitized samples of the waveforms are input to an
input serial-to-parallel logic 117 for conversion to a parallel
array of time domain data. A Fast Fourier Transform logic 119
converts the data array back to complex numbers, which are then
mapped by the differential decoder and slicer 121 to corresponding
bit sequences to recreate the original bit stream. Errors in the
bit stream or loss of bits can be minimized through the use of two
different frequency diversity techniques.
It should be apparent to one of ordinary skill in the art that the
invention applies to different types of information carried by the
analog signals transmitted. These include time marks, reference
symbols, and user data symbols themselves.
As shown in FIG. 2, the transmitter side AFE circuit 109 receives a
digital symbol at its digital to analog (D/A) convertor 201, which
outputs a corresponding analog waveform. The analog waveform is
then filtered via filter 203. To provide for frequency diversity,
spectral images on the harmonics of the carrier frequency can be
manipulated by increasing or decreasing the sampling rate of the
D/A convertor 221 so that the harmonic images fall under the filter
range of filter 223. These images are then demodulated by
demodulator 209. The demodulator 209 has a mixer 205, which
multiplies the filtered signal with a clock signal from clock 207.
The above arrangement advantageously permits recovery of the
transmitted symbol by the receiver 101 despite noisy channel
conditions due to signal reflections and electrical noise.
The DMT system of the present invention inherently provides
tolerance to noise in part because of the use of differential
encoding. FIG. 3 illustrates the general principle of this coding
scheme, where the graphs 301 and 303 represent a previously sent
symbol and the bottom graphs represent the current symbol that is
to be transmitted over the chanrel 125. For example, assume the
transmitter circuit 101 receives a bit stream in which 01 is the
first bit sequence. Under this exemplary scheme, the bit sequence
is 2 bits in length; however, N bits can be used in which the
number of constellation points would equal 2.sup.N. The transmitter
first sends a reference symbol, which assigns a reference point on
the complex plane for each tone. In this particular example, the
reference point of this tone is point A (graph 301). Because of the
channel response, the reference point may be received as point A',
shown in graph 303. Now, assume that the mapping of the bit
sequence 01 is to point B (on graph 305), which is a 90.degree.
phase shift. That is, a 90.degree. phase shift represents bits 01.
The symbol is thus encoded accordingly. Channel characteristics
remain nearly constant between symbol transmissions. As a result,
the amplitude and phase relation between two constellation points
should stay reasonably constant, thereby permitting the encoding of
information in the relative phase position. Once across the
channel, the received constellation point is point B', in which the
receiver circuit 111 detects a 90.degree. phase difference and an
amplitude difference of 0. The 90.degree. phase difference
indicates to the receiver 111 that point B' should be remapped or
decoded as 01. By differentially encoding a signal, information
about the channel characteristics are not needed, thus minimizing
the impact of channel response. This is possible, in part, because
channel characteristics do not change too abruptly. Furthermore,
differentially encoding the bit sequence on a consecutive symbol
symbol basis, as opposed to using a fixed reference symbol, is
equivalent to updating the constellation rotation, thereby reducing
problems associated with transmitter and receiver clock
difference.
FIGS. 4a and 4b represent two alternative embodiments of the AFE
circuit 109 of FIG. 2 for frequency shifting the symbol by
multiplying the clock signal (CLK), which has a frequency of
f.sub.CLK, with the symbol waveform (i.e., baseband signal). The
clock 423, in an exemplary embodiment, serves as the system clock
for both the transmitter circuit 101 and receiver circuit (not
shown) within a DMT transceiver (not shown). FIG. 4a illustrates
one embodiment of AFE circuit 109, in which a digital mixer 413 up
converts the baseband signal to a passband signal around f.sub.CLK
according to the following frequency component equations:
Per equation (1), Y(f) is the output of the convolution of the
baseband signal (or modulating signal), F(f), by the clock signal
C(f) running at carrier frequency f.sub.CLK. As evident from
equation (2), the frequency component of the clock signal, C(f), is
an infinite series of impulses with amplitudes .alpha..sub.n. The
convolution of F(f) and C(f) yields equation (3), which indicates
that the frequency spectrum of the baseband signals are spaced
according to the clock rate, f.sub.CLK. The implementation of
equation (3) is accomplished by an embodiment of the AFE circuit
109 that employs digital mixer 413, which comprises a repeating
samples block 401 and an alternating sign block 403. The repeating
samples block 401, as the name suggests, duplicates or repeats the
baseband signal based upon the clock rate f.sub.CLK. Such repeating
samples alternately undergo a sign change via the alternating sign
block 403. For example, if the first sample has a positive
amplitude, the next sample would be made negative; this assignment
occurs for all samples of a digital waveform. Ultimately, the
alternating sign block 403 outputs the digital waveform to a D/A
converter 407. It should be noted that the repeating samples block
401, alternating sign block 403, and the D/A converter 407 all rely
on the clock 405 to properly process the waveforms. The D/A
converter 407 outputs an analog waveform to be filtered by bandpass
filter 409. This embodiment of the AFE circuit 109 provides a
simple way to multiply the clock signal with the baseband digital
waveform in that the components of the digital mixer 411 do not
necessitate complex logic. Another embodiment of the AFE circuit
109 that implements frequency shifting based upon the clock signal
is shown in FIG. 4b.
FIG. 4b provides a technique for frequency shifting a symbol
through the use of an analog switch 421 that is driven by clock
405. Like the arrangement of FIG. 4a, the principle of operation of
this particular embodiment is governed by equations (1)-(3). Under
the current configuration, the up conversion is accomplished via
analog switch 421, which is deployed behind the D/A converter 201.
That is, the digital waveform of the symbol is first converted to
its analog equivalent and then processed. The operation of the
analog switch 421 follows the binary values of CLK. When CLK is
HIGH, the analog switch 421 is ON; correspondingly, when the clock
signal is LOW, the analog switch 421 is OFF. The switching of the
analog signals yields a spectrum that is the same as the previous
embodiment. This spectrum is shown in FIG. 5.
FIG. 5 illustrates an exemplary spectrum of the symbol to be
transmitted by the transmitter circuit 101. The baseband signal of
the symbol has a spectral image 500, which is then mixed using
either of the embodiments of the AFE circuit 109 discussed with
respect to FIGS. 4a and 4b. After the up conversion using the clock
405, the spectrum of 520 results. The figure reveals that the
spectral image 500 has been frequency repeated and shifted by the
clock rate f.sub.CLK ; i.e., at -f.sub.CLK and +f.sub.CLK. The
envelope 511 of the bandpass filter 409 passes the images 501, 503,
507, and 509 and eliminates image 505. The filter 409 can be
readily relaxed to capture more harmonics to provide frequency
diversity. In other words, the bandwidth of the filter 409 can be
increased as appropriate. Assuming that the channel completely
distorts the images 501 and 509 at the carrier frequency
(i.e.,f.sub.CLK and +f.sub.CLK) such that the power spectral
densities of these images 501 and 509 are negligible, the encoded
information is not lost. Such information is preserved in the
harmonics 503 and 507. Furthermore, by proper sampling on the
receiver side, the spectral images can be made to overlap as in
graph 530. In this manner, more harmonic images can be captured by
the filter 409 to guard against the possibility of losing the image
at the carrier frequency. For example, if the spectral image 501 is
eliminated, image 503 can be demodulated.
According to the disclosed embodiments, a DMT system using
differential encoding employs two techniques to frequency shift
using a system clock signal. One approach utilizes an AFE circuit
with a digital mixer to up convert the baseband symbol to the clock
rate. Another effective arrangement manipulates the symbol in
analog form with an analog switch that switches ON and OFF in
response to the clock signal. These techniques enable manipulation
of the spectral characteristics of the transmitted symbol to that
the effects of channel impairments can be minimized. Because of the
elegance of these two approaches, component cost can be
reduced.
While this invention has been described in connection with what is
presently considered to be the most practical and preferred
embodiments, it is to be understood that the invention is not
limited to the disclosed embodiments, but, on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
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